Apparatus for treating intervertebral discs with resistive energy

ABSTRACT

An externally guidable intervertebral disc apparatus manipulates disc tissue at a selected location of an intervertebral disc. The apparatus comprises a catheter having a distal end, a proximal end and a longitudinal axis. The catheter has an intradiscal section at its distal end. The intradiscal section is extendible into the disc, has sufficient rigidity to be advanceable through the nucleus pulposus and around the annulus fibrosus of the disc under a force applied longitudinally to the proximal end, has sufficient flexibility in a direction of the disc plane to be compliant with the inner wall, and has insufficient penetration ability to be advanceable out through the annulus fibrosus under the applied force. A thermal energy delivery device is located in the intradiscal section for preferentially heating the selected location of the disc. The apparatus is used to locally and controllably heat but not char or burn tissue in a disc. Such heat shrinks the collagen component.

REFERENCES TO PARENT AND CO-PENDING APPLICATIONS

U.S. Provisional Application Nos. 60/029,600, 60/029,602, 60/029,734, 60/029,735, filed Oct. 23, 1996; and U.S. Provisional Application No. 60/047,818, filed May 28, 1997; application Ser. No. 08/881,692 entitled "Method and Apparatus for Treating Intervertebral Discs with Electromagnetic Energy", filed on Jun. 24, 1997; application Ser. No. 08/881,527 entitled "Method and Apparatus for Delivering and Removing Material from the Interior of an Intervertebral Discs", filed on Jun. 24, 1997; application Ser. No. 08/881,693 entitled "Method and Apparatus for Treating Annular Fissures in Intervertebral Discs" filed on Jun. 24, 1997; and application Ser. No. 08/881,694 entitled "Method and Apparatus for Treating Intervertebral Disc Degeneration", filed on Jun. 24, 1997, are hereby incorporated by reference.

BACKGROUND

1. Field of the Invention

This invention relates to methods and apparatuses to treat intervertebral disc problems using percutaneous techniques without the need for major surgical intervention and more particularly to modify intervertebral disc tissue using controlled thermal energy, especially a resistive heater.

2. Description of Related Art

Intervertebral disc abnormalities have a high incidence in the population and may result in pain and discomfort if they impinge on or irritate nerves. Disc abnormalities may be the result of trauma, repetitive use, metabolic disorders and the aging process and include such disorders but are not limited to degenerative discs with (i) localized tears or fissures in the annulus fibrosus, (ii) localized disc herniations with contained extrusions, and (iii) chronic, circumferential bulges.

Disc fissures and tears result from structural degeneration (a part of the aging process that may be accelerated by trauma) of fibrous components of the annulus fibrosus. Even sneezing, bending or just attrition can tear these degenerated annulus fibers, creating a fissure. The fissure itself may be the sole morphological change, above and beyond generalized degenerative changes in the connective tissue of the disc. Biochemicals contained within the nucleus pulposus are alleged to escape through the fissure and irritate nearby structures. Fissures of the annulus can be debilitatingly painful. The fissure may be associated with a herniation of that portion of the annulus.

With a contained disc herniation, the nucleus pulposus works its way partly through the wall but is still contained within the annulus or beneath the posterior longitudinal ligament, and there are no free disc fragments in the spinal canal. Nevertheless, even a contained disc herniation is problematic because the outward protrusion can press on the spinal nerves or irritate other structures.

Another disc problem occurs when the disc bulges outward circumferentially in all directions and not just in one location. This occurs when over time the disc weakens, bulges outward and takes on a "roll" shape. Mechanical stiffness of the joint is reduced and the joint may become unstable. One vertebra may eventually settle on top of another. This problem continues as the body ages and accounts for shortened stature in old age. With the increasing life expectancy of the population, such degenerative disc disease and impairment of nerve function are becoming major public health problems. As the disc "roll" extends beyond the normal circumference, the disc height may be compromised which compresses vertebral foramina with sensitive nerves. In addition, osteophytes may form on the outer surface of the disc roll and further encroach on the spinal canal and nerve foramina. This condition is called lumbar spondylosis.

It has been thought that disc degeneration causes pain predominantly via segmental instability which disturbs sensitive structures which in turn register pain. Traditional, conservative methods of treatment include bed rest, pain and muscle relaxant medication, physical therapy and steroid injection. Upon failure of conservative therapy, spinal pain (assumed to be due to instability) has been treated by spinal fusion, which causes the vertebrae above and below the disc to grow solidly together and form a single, solid piece of bone. Fusion may be performed with or without instrumentation. Other treatments include discectomy alone or disc decompression with or without fusion. Disc decompression is performed to reduce pressure on the annulus (and its outward protrusion) by removing some of the nucleus contents by percutaneous nuclectomy. Surgical complications include disc space infection, nerve root injury, hematoma formation, collapse of the disc from decrease in height, and instability of the adjacent vertebrae.

These interventions have been problematic in that alleviation of back pain is unpredictable even if surgery is successful. In attempts to overcome these difficulties, new devices have been introduced to the market, including but not limited to pedicle screws and interbody fusion cages. Although pedicle screws provide a high fusion success rate, there is still no direct correlation between fusion success rate and patient improvement in function and pain. Studies on fusion have demonstrated success rates of between 50% and 67% for pain improvement. And a significant number of patients have more pain postoperatively. Therefore, different methods of helping patients with degenerative disc problems need to be explored.

FIGS. 1(a) and 1(b) illustrate a cross-sectional anatomical view of a vertebra and associated disc and a lateral view of a portion of a lumbar and thoracic spine, respectively. Structures of a typical cervical vertebra (superior aspect) are shown in FIG. 1(a): 104--lamina; 106--spinal cord; 108--dorsal root of spinal nerve; 114--ventral root of spinal nerve; 116--posterior longitudinal ligament; 118--intervertebral disc; 120--nucleus pulposus; 122--annulus fibrosus; 124--anterior longitudinal ligament; 126--vertebral body; 128--pedicle; 130--vertebral artery; 132--vertebral veins; 134--superior articular facet; 136--posterior lateral portion of the annulus; 138--posterior medial portion of the annulus; 142--spinous process; and 144--dura mater. In FIG. 1(a), half of the intervertebral disc 118 has been cut away so that half of the vertebral body 126 can be seen. FIG. 1(b) is a lateral aspect of the lower portion of a typical spinal column showing the entire lumbar region and part of the thoracic region showing the following structures: 118--intervertebral disc; 126--vertebral body; 142--spinous process; 168--inferior articular process; 170--inferior vertebral notch; 110--spinal nerve; 174--superior articular process; 176--lumbar curvature; and 180--sacrum.

The presence of the spinal cord or nerve sac and the posterior portion of the vertebral body, including the spinous process, lamina and superior and inferior articular processes, prohibit introduction of a needle or trocar from a directly posterior position. This is important because the posterior disc wall is the site of symptomatic annulus tears or disc protrusions/extrusions that compress or irritate spinal nerves for most degenerative disc syndromes. The inferior articular process 168, along with the pedicle 128 and the lumbar spinal nerve 110, form a small "triangular" window (shown in black in FIG. 1(c)) through which needle introduction can be achieved from the posterior lateral position. FIG. 1(d) is a cross-sectional view of the lower back with an instrument introduced by the posterior lateral approach. It is well known to those skilled in the art that percutaneous access to the disc is achieved by placing an introducer into the disc from this posterior lateral approach, but the triangular window does not allow much room to maneuver. Once the introducer pierces the tough annulus fibrosus, the introducer is fixed at two points along its length and has very little freedom of movement. Thus, this approach has allowed access only to small central and anterior portions of the nucleus pulposus. Current methods do not permit percutaneous access to the posterior half of the nucleus or to the posterior wall of the disc. Major and potentially dangerous surgery would be required to access these areas.

U.S. Pat. No. 5,433,739 (the "'739 patent") discloses placement of an RF electrode in an interior region of the disc approximately at the center of the disc. RF power is applied, and heat then putatively spreads out globally throughout the disc. The '739 patent teaches the use of a rigid shaft which includes a sharpened distal end that penetrates through the annulus fibrosus and into the nucleus pulposus. In one embodiment the shaft has to be rigid enough to permit the distal end of the RF electrode to pierce the annulus fibrosus, and the ability to maneuver its distal end within the nucleus pulposus is limited. In another embodiment, a somewhat more flexible shaft is disclosed. However, the embodiments of the '739 patent do not permit access to the posterior, posterior lateral and posterior medial region of the disc, nor do they provide for focal delivery of therapy to a selected local region within the disc or precise temperature control at the annulus. The '739 patent teaches the relief of pain by globally heating the disc. It does not disclose contraction or tightening of a herniated or bulging disc or treating of an annular fissure.

U.S. Pat. No. 5,201,729 (the "'729 patent") discloses the use of an optical fiber that is introduced into a nucleus pulposus. In the '729 patent, the distal end of a stiff optical fiber shaft extends in a lateral direction relative to a longitudinal axis of an introducer. This prevents delivery of coherent energy into the nucleus pulposus in the direction of the longitudinal axis of the introducer. Due to the limited access from the posterior lateral approach, stiff shaft, and lateral energy delivery, the device of the '729 patent is unable to gain close proximity to any selected portion of the annulus (i.e., posterior, posterior medial and central posterior locations) requiring treatment or to precisely control temperature in the nucleus at this location. The '729 patent teaches the use of coherent energy to vaporize the nucleus pulposus, which by the nature of the technique generates very high temperatures. No treatment of annular herniations or disc bulges is disclosed.

Accordingly, it is desirable to treat disc abnormalities at locations previously not accessible via percutaneous procedures without substantial destruction to the disc. It would be further desirable to provide controllable thermal energy to the posterior nucleus pulposus and the posterior, posterior lateral and the posterior medial regions of the inner wall of the annulus fibrosus without heating other regions of the nucleus, as would occur with prior art heating elements.

SUMMARY OF THE INVENTION

Accordingly, an object of the invention is to provide a minimally invasive method and apparatus for treating morphologic abnormalities of discs by delivery of controlled thermal energy at selected locations within the disc.

Another object of the invention is to provide an apparatus which is advanceable and navigational around the inner wall of the annulus fibrosus for delivery of such thermal energy.

Still a further object of the invention is to provide a device which has a distal end that is inserted into the disc and accesses the posterior, posterior lateral and the posterior medial regions of the inner wall of the annulus fibrosus to provide localized heating at such a location.

These and other objects of the invention as will hereafter become more apparent have been accomplished by providing an externally guidable intervertebral disc apparatus for manipulation of tissue at a selected location of an intervertebral disc. The apparatus comprises a catheter having a distal end, a proximal end and a longitudinal axis. The catheter has an intradiscal section at the distal end of the catheter. The intradiscal section is extendible into the disc, has sufficient rigidity to be advanceable through the nucleus pulposus and around the inner wall of the annulus fibrosus of the disc under a force applied longitudinally to the proximal end, has sufficient flexibility in a direction of the disc plane to be compliant with the inner wall, has insufficient penetration ability to be advanceable out through the annulus fibrosus under the applied force. The apparatus also comprises a thermal energy device located in the intradiscal section for preferentially heating the selected location of the disc.

Also disclosed is an intervertebral disc apparatus which comprises a catheter including an intradiscal section with a thermal energy delivery device, wherein the intradiscal section has sufficient column strength to be advanceable through the nucleus pulposus of an intervertebral disc and to be navigable along the inner wall of the annulus fibrosus, a low enough flexural strength in an axial plane of the intervertebral disc, and a sufficient flexural strength in oblique and cephalo-caudal planes to be constrained within the axial plane.

Also provided is a method of manipulating disc tissue at a selected location of an intervertebral disc, the disc having a nucleus pulposus and an annulus fibrosus, the annulus fibrosus having an inner wall, and the nucleus pulposus having a diameter in a disc plane between opposing sections of the inner wall. The method comprises providing a catheter having a thermal energy device element, a distal end, a proximal end and a longitudinal axis; applying a force longitudinally to the proximal end of the catheter which is sufficient to advance the catheter through the nucleus pulposus and around the inner wall but which force is insufficient to cause the catheter to puncture the annulus; positioning the thermal energy delivery device at the selected location of the disc by advancing or retracting the catheter and optionally twisting the proximal end of the catheter; and heating the selected location of the disc with the thermal energy delivery device. Such a method shrinks the annulus fibrosus without charring or vaporizing tissue.

BRIEF DESCRIPTION OF THE FIGURES

The invention will be better understood by reference to the following figures that form part of the current specification, wherein:

FIG. 1(a) is a superior cross-sectional anatomical view of a cervical disc and vertebra.

FIG. 1(b) is a lateral anatomical view of a portion of the spine.

FIG. 1(c) is a posterior-lateral anatomical view of two lumbar vertebrae and illustration of the triangular working zone.

FIG. 1(d) is a superior cross-sectional view of the required posterior lateral approach.

FIG. 2 is a second cross-sectional view of an intervertebral disc illustrating a disc plane of the intervertebral disc and an oblique or cephalo-caudal plane.

FIG. 3(a) is a plan view of an introducer and an instrument of the invention in which solid lines illustrate the position of the instrument in the absence of bending forces and dotted lines indicate the position the distal portion of the instruments would assume under bending forces applied to the intradiscal section of the instrument.

FIG. 3(b) is an end view of the handle of the embodiment shown in FIG. 3(a).

FIG. 4 is a cross-sectional view of an intervertebral disc with a portion of the intervertebral apparatus of the present invention inserted in the intervertebral disc.

FIG. 5(a) is a cross-sectional view of the intervertebral segment of the embodiment of the invention shown in FIG. 3(a) taken along the line 5(a)--5(a) of FIG. 3(a).

FIG. 5(b) is a cross-sectional view of the intervertebral segment of a second embodiment of the present invention having a combined wall/guiding mandrel.

FIG. 6 is a perspective view of an embodiment of an apparatus of the present invention with a resistive heating coil positioned around an exterior of an intradiscal section of the catheter.

FIG. 7 is a partial cross-sectional view of an embodiment of an apparatus of the invention illustrating a sensor positioned in an interior of the intradiscal section of the catheter.

FIG. 8 is a partial cross-sectional view of an embodiment of the apparatus of the invention further including a sheath positioned around the resistive heating coil.

FIG. 9 is a partial cross-sectional view of an embodiment of the apparatus of FIG. 6 with multiple resistive heating coils.

FIG. 10 is a plan view of an embodiment of the intradiscal section of a catheter of the invention made with a helical structure.

FIG. 11 is a block diagram of an open or closed loop feedback system that couples one or more sensors to an energy source.

FIG. 12 is a block diagram of an embodiment illustrating an analog amplifier, analog multiplexer and microprocessor used with the feedback control system of FIG. 11.

The invention now being generally described, the same will be better understood by reference to the following detailed description of specific embodiments and general features of the invention.

DETAILED DESCRIPTION

The present invention provides a method and apparatus for treating intervertebral disc disorders by the application of controlled heating to a localized region of the disc, with or without desiccation. Such disorders include but are not limited to degenerative discs which with (i) localized tears or fissures in the annulus fibrosus, (ii) localized disc herniations with contained extrusions, and (iii) chronic, circumferential bulges.

In general, an apparatus of the invention is in the form of an externally guidable intervertebral disc apparatus with a functional element that allows heating of disc tissue present at a selected location of an intervertebral disc. Use of resistive heating, with its characteristic of fine temperature control, combined with the positional control of the catheter of the invention, allows preferential heating of selected location(s) within the disc. For ease of reference to various manipulations and distances described below, the nucleus pulposus can be considered as having a given diameter in a disc plane between opposing sections of the inner wall. The nucleus pulposus diameter measurement allows instrument sizes (and parts of instruments) designed for one size disc to be readily converted to sizes suitable for an instrument designed for a different size of disc.

The operational portion of the apparatus of the invention is brought to a location in or near the disc using techniques and apparatuses typical of percutaneous interventions. For convenience and to indicate that the apparatus of the invention can be used with any insertional apparatus that provides proximity to the disc, including many such insertional apparatuses known in the art, the term "introducer" is used to describe this aid to the method. An introducer has an internal introducer lumen with a distal opening at a terminus of the introducer to allow insertion (and manipulation) of the operational parts of the apparatus into (and in) the interior of a disc.

The operational part of the apparatus comprises an elongated element referred to as a catheter, various parts of which are located by reference to a distal end and a proximal end at opposite ends of its longitudinal axis. The proximal end is the end closest to the external environment surrounding the body being operated upon (which may still be inside the body in some embodiments if the catheter is attached to a separate handle insertable into the introducer). The distal end of the catheter is intended to be located under conditions of use inside the disc. The catheter is not necessarily a traditional medical catheter (i.e., an elongate tube for admission or removal of fluids from an internal body cavity location), but is a defined term for the purposes of this specification. "Catheter" has been selected as the operative word to describe this part of the apparatus, as the inventive apparatus is a long flexible tube which transmits thermal energy from a location external to the body to a location internal to the disc to provide readily controllable heating at the desired location, with or without desiccation.

The catheter is adapted to slidably advance through the introducer lumen, the catheter having an intradiscal section at the distal end of the catheter, the intradiscal section being extendible through the distal opening at the terminus of the introducer into the disc. Although the length of the intradiscal portion can vary with the intended function as explained in detail below, a typical distance of extension is at least one-half the diameter of the nucleus pulposus, preferably in the range of one-half to one and one-half times the circumference of the nucleus pulposus.

For a resistance heating element to be guided to the desired location within a disc, the intradiscal portion of the catheter is manufactured with sufficient rigidity to avoid collapsing upon itself while being advanced through the nucleus pulposus of the disc and while being navigated around the inner wall of the annulus fibrosus. The intradiscal portion, however, has insufficient rigidity to puncture the annulus fibrosus under the force applied to advance the catheter through the nucleus pulposus and then navigate around the inner wall. Absolute penetration ability will vary with sharpness and stiffness of the tip of the catheter, but in all cases a catheter of the present invention will advance more readily through the nucleus pulposus than through the annulus fibrosus.

In preferred embodiments, the intradiscal section of the catheter further has differential bending ability in two orthogonal directions at right angles to the longitudinal axis. This causes the catheter to bend along a desired plane (instead of at random). Also when a torsional (twisting) force is applied to the proximal end of the catheter, the distal end of the catheter is re-oriented in a controlled direction and navigation of the catheter in a desired plane is possible.

A further component of the catheter is a functional element located in the intradiscal section for providing heat at the selected location of the disc where some therapeutic action is desired. The apparatus allows the functional element to be controllably guided by manipulation of the proximal end of the catheter into a selected location for localized application of heat to a degenerate or injured portion of the disc.

The method of the invention, which involves manipulating a disc tissue present at a selected location of an intervertebral disc, is easily carried out with an apparatus of the invention. An introducer is provided located in a body so that its proximal end is external to the body being operated upon and the distal opening of its lumen is internal to the body and (1) internal to the annulus fibrosus or (2) adjacent to an opening in the annulus fibrosus leading to the nucleus pulposus, such as an annular tear that communicates with the nucleus pulposus or an opening created by a trocar expressly for the purpose of advancing the apparatus of the invention. The catheter is then slid into position in and through the introducer lumen so that the functional element in the catheter is positioned at the selected location of the disc by advancing or retracting the catheter in the introducer lumen and optionally twisting the proximal end of the catheter to precisely navigate the catheter. By careful selection of the rigidity of the catheter and by making it sufficiently blunt to not penetrate the annulus fibrosus, and by careful selection of the flexibility in one plane versus the orthogonal plane, the distal portion of the catheter will curve along the inner wall of the annulus fibrosus as it is advanced and is selectively guided to position(s) within the disc. Thermal energy is then applied at the selected location(s) of the disc with the resistive heating element in a controlled amount to shrink tissue, with or without desiccation.

Each of the elements of the apparatus and method will now be described in more detail. However, a brief description of disc anatomy is provided first, as sizes and orientation of structural elements of the apparatus and operations of the method can be better understood in some cases by reference to disc anatomy.

The annulus fibrosus is comprised primarily of tough fibrous material, while the nucleus pulposus is comprised primarily of an amorphous colloidal gel. There is often a transition zone between the annulus fibrosus and the nucleus pulposus made of both fibrous material and amorphous colloidal gel. The border between the annulus fibrosus and the nucleus pulposus becomes more difficult to distinguish as a patient ages, due to degenerative changes. This process may begin as early as 30 years of age. For purposes of this specification, the inner wall of the annulus fibrosus can include the young wall comprised primarily of fibrous material as well as the transition zone which includes both fibrous material and amorphous colloidal gels (hereafter collectively referred to as the "inner wall of the annulus fibrosus"). Functionally, the location at which there is an increase in the resistance to catheter penetration sufficient to cause a bending of the distal portion of the catheter into a radius less that of the external wall of the annulus fibrosus is considered the inner wall of the annulus fibrosus.

As with any medical instrument and method, not all patients can be treated, especially when degeneration or injury is too severe. There is a medical gradation of degenerative disc disease (stages 1-5). See, for example, Adams et al., "The Stages of Disc Degeneration as Revealed by Discograms," J. Bone and Joint Surgery, 68, 36-41 (2086). As these grades are commonly understood, the methods of instrument navigation described herein would probably not be able to distinguish between the nucleus and the annulus in degenerative disease of grade 5. In any case, most treatment would occur for a disc in stages 3 and 4, as stages 1 and 2 are asymptomatic in most patients, and stage 5 may require disc removal and fusion.

Some of the following discussion refers to motion of the catheter inside the disc by use of the terms "disc plane," "oblique plane" and "cephalo-caudad plane." These specific terms refer to regions of the intervertebral disc. Referring now to FIG. 2 (which shows a vertical cross-section of a disc), a disc plane 30 of the intervertebral disc is generally a plane of some thickness 27 within the nucleus pulposus 120 orthogonal to the axis formed by the spinal column (i.e., such a disc plane would be substantially horizontal in a standing human, corresponding to the "flat" surface of a vertebra. An oblique plane 31 extends along any tilted orientation relative to axial plane 30. However, when the plane is tilted at 90°, such a plane would be substantially vertical in a standing human, and is referred to as a cephalo-caudal plane. Reference is made to such planes to describe navigation of the catheter away from the disc plane. In various embodiments, disc plane 30 has a thickness no greater than the thickness of the intervertebral disc, preferably a thickness no greater than 75% of a thickness of the intervertebral disc, and more preferably a thickness no greater than 50% of a thickness of the intervertebral disc. Movement of the intradiscal portion 16 of catheter 14 is confined within a disc plane by the physical and mechanical properties of the intradiscal portion 16 during advancement of the catheter when the bending plane of the catheter is aligned with the disc plane, until some additional force is applied to the catheter by the physician. A twisting force (which can be applied mechanically, electrically, or by any other means) acting on the proximal end of the catheter changes the forces acting on the distal end of the catheter so that the plane of the bend of the catheter can be angled relative to the disc plane as the catheter is advanced. Thus, the distal end of the catheter can be moved up or down (depending on the direction of the twist) within the intervertebral disc under the control of the physician.

Turning now to the introducer apparatus, a detailed description of an entire apparatus should not be necessary for those skilled in the art of percutaneous disc procedures and the design of instruments intended for such use. The method of the invention can also be carried out with endoscopic instruments, and an endoscopic apparatus having structural parts that meet the descriptions set forth in this specification would also be an apparatus of the invention.

In general, a device of the invention can be prepared in a number of different forms and can consist (for example) of a single instrument with multiple internal parts or a series of instruments that can be replaceable and sequentially inserted into a hollow fixed instrument (such as a needle) that guides the operational instruments to a selected location within or adjacent to an intervertebral disc. Because prior patents do not fully agree on how to describe parts of percutaneous instruments, terminology with the widest common usage will be used.

The introducer, in its simplest form, can consist of a hollow needle-like device (optionally fitted with an internal removable obturator or trocar to prevent clogging during initial insertion) or a combination of a simple exterior cannula that fits around a trocar. The result is essentially the same: placement of a hollow tube (the needle or exterior cannula after removal of the obturator or trocar, respectively) through skin and tissue to provide access into the annulus fibrosus. The hollow introducer acts as a guide for introducing instrumentation. More complex variations exist in percutaneous access instruments designed for other parts of the body and can be applied to design of instruments intended for disc operations. Examples of such obturators are well known in the art. A particularly preferred introducer is a 17- or 18-gauge, thin-wall needle with a matched obturator, which after insertion is replaced with a catheter of the present invention.

Referring now to the figures, FIGS. 3(a) and 3(b) illustrate one embodiment of a catheter 14 of the invention as it would appear inserted into an introducer 12. The apparatus shown is not to scale, as an exemplary apparatus (as will be clear from the device dimensions below) would be relatively longer and thinner; the proportions used in FIG. 3(a) were selected for easier viewing by the reader. The distal portion of an intervertebral apparatus operates inside an introducer 12 having an internal introducer lumen 13. A flexible, movable catheter 14 is at least partially positionable in the introducer lumen 13. Catheter 14 includes a distal end section 16 referred to as the intradiscal section, which is designed to be the portion of the catheter that will be pushed out of the introducer lumen and into the nucleus pulposus, where movement of the catheter will be controlled to bring operational portions of the catheter into the proper location within the disc. In FIG. 3(a), dashed lines are used to illustrate bending of the intradiscal portion of the catheter as it might appear in use, as discussed in detail later in the specification. FIG. 3(b) shows an end view of a handle 11 at the proximal end of the catheter, with the handle 11 having an oval shape to indicate the plane of bending, also discussed in detail later in the specification. Other sections and properties of catheter 14 are described later.

For one embodiment suitable for intervertebral discs, the outer diameter of catheter 14 is in the range of 0.2 to 5 mm, the total length of catheter 14 (including the portion inside the introducer) is in the range of 10 to 60 cm, and the length of introducer 12 is in the range of 5 to 50 cm. For one preferred embodiment, the catheter has a diameter of 1 mm, an overall length of 30 cm, and an introduced length of 15 cm (for the intradiscal section). With an instrument of this size, a physician can insert the catheter for a distance sufficient to reach any selected location in the nucleus of a human intervertebral disc.

FIG. 4 illustrates the anatomy of an intervertebral disc and shows an apparatus of the invention inserted into a disc. Various structures of the disc are identified by anatomical designations: posterior lateral inner annulus 136, posterior medial inner annulus 138, annulus fibrosus 122/nucleus pulposus 122 interface (or inner wall of the annulus 22), annulus/dural interface 146, annulus/posterior longitudinal ligament interface 148, anterior lateral inner annulus 150, and anterior medial inner annulus 152.

Referring again to FIG. 4, the mechanical characteristics of intradiscal section 16 of catheter 14 are selected to have (1) sufficient column strength along the longitudinal axis of the catheter to avoid collapse of the catheter and (2) different flexural strengths along the two axes orthogonal to that longitudinal axis to allow controlled bending of the catheter. These parameters make the catheter conformable and guidable along inner wall 22 of an annulus fibrosus 122 to reach selected locations, such as the posterior medial annulus 138.

Specific mechanical characteristics of particular designs will be described later in the examples that follow. Generally, however, the necessary design features can be selected (in an interrelated fashion) by first providing the intradiscal section of the catheter with sufficient column strength to be advanceable through normal human nucleus pulposus material and around the inner wall of the annulus without collapse. Here "collapse" refers to bending sufficient to inhibit further advancement of the tip. Advancement of the tip is restricted by a) sliding through the normal gelatinous nucleus pulposus, b) encountering fibrous clumps of the nucleus pulposus, and c) curving and advancing along the inner wall of the annulus fibrosus. Column strength can be increased in many ways known in the art, including but not limited to selecting materials (e.g., metal alloy or plastic) with a high resistance to bending from which to form the catheter, forming the structure of the catheter with elements that add stiffening (such as bracing), and increasing the thickness of the structural materials. Column strength can be decreased to favor bending by selecting the opposite characteristics (e.g., soft alloys, hinging, and thin structural elements).

When the catheter collapses, the physician feels an abrupt decrease in resistance. At that time the catheter forms one or more loops or kinks between the tip of the introducer and the distal tip of the catheter.

In some embodiments, the tip 28 of intradiscal section 16 is biased or otherwise manufactured so that it forms a pre-bent segment prior to contact with the annulus fibrosus as shown in FIG. 3(a). The bent tip will cause the intradiscal section to tend to continue to bend the catheter in the same direction as the catheter is advanced. This enhanced curving of a pre-bent catheter is preferred for a catheter that is designed to reach a posterior region of the nucleus pulposus; however, such a catheter must have sufficient column strength to prevent the catheter from collapsing back on itself.

The intradiscal section not only must allow bending around the relatively stronger annulus fibrosus in one direction, but also resist bending in the orthogonal direction to the plane in which bending is designed to occur in. By twisting the proximal end of a catheter and thus controlling the orientation of the plane of bending while concurrently controlling the advancement of the catheter through the nucleus, a physician can navigate the catheter and its instrumentation within the disc.

The bending stiffness of the intradiscal section as measured in Taber stiffness units (using a length of the inventive catheter as the test strip rather than the standard-dimension, homogeneous-material test strip) should be in the range of 2-400 units (in a 0-10,000 unit range) in the desired bending plane, preferably 3-150 units. In preferred embodiments, stiffness is in the range of 4-30 units in the desired bending plane. In all cases, the bending stiffness preferably is 2-20 times higher for bending in the orthogonal direction.

The column or compressive strength of the intradiscal section (force required to buckle a segment whose length is 25 or more times its diameter) is in the range of 0.05 to 4 kg, preferably 0.05 to 2 kg. In the most preferred embodiments, it is in the range of 0.25-2 lbs. In the proximal shaft section (i.e., the part of the catheter proximal to the intradiscal section), this strength is in the range of 0.1 to 25 kg, preferably 0.2 to 7 kg. In the most preferred embodiments, it is in the range of 0.7 to 4 kg.

Returning now to FIG. 4, intradiscal section 16 is guidable and can reach the posterior, the posterior lateral 136, and the posterior medial 138 regions of the posterior wall of the annulus fibrosus, as well as any other selected section on or adjacent to inner wall 22. In order to move the functional section of the catheter with the resistive heating element into a desired nucleus location, intradiscal section 16 is first positioned in the nucleus pulposus 120 by means of the introducer 12.

In most uses, introducer 12 pierces annulus fibrosus 122 and is advanced through the wall of the annulus fibrosus 122 into the nucleus pulposus 120. In such embodiments, introducer 12 is then extended a desired distance into nucleus pulposus 120. Catheter 14 is advanced through a distal end of introducer 12 into nucleus pulposus 120. Advancement of the catheter 14, combined with increased resistance to advancement into the annulus fibrosus, causes the tip of the intradiscal section to bend relative to the longitudinal axis of introducer 12 when the distal portion 28 of intradiscal section 16 contacts the inner wall of the annulus fibrosus. Catheter 14 is navigated along inner wall 22 of annulus fibrosus 122 to any selected site on inner wall 22 or within nucleus pulposus 120. For example, intradiscal section 16 can be positioned on or adjacent to a fissure or tear 44 of annulus fibrosus 122.

The distal portion 28 of intradiscal section 16 is designed to be incapable of piercing annulus fibrosus 122. The inability of distal portion 28 to puncture the annulus can be the result of either the shape of the tip 29 or flexibility of distal portion 28, or both. The tip 29 is considered sufficiently blunt when it does not penetrate the inner wall of the annulus fibrosus but is deflected back into the nucleus pulposus or to the side around the inner wall when the tip 29 is advanced. The tip can be made with a freely rotating ball. This embodiment provides not only a blunt surface but also a rolling contact to facilitate navigation.

Many percutaneous and endoscopic instruments designed for other purposes can be adapted for use in this invention. This permits other functions (such as cutting and grasping) to be performed at the desired location after the catheter is advanced to that position. For example, cutting edges and sharp points can be present in the distal portion 28 if they can be temporarily masked by a covering element. However, such other operative devices must be sufficiently flexible and thin to meet the design characteristics described in this specification.

In another embodiment an introducer 12 pierces the skin and reaches an exterior of annulus fibrosus 122. A rigid and sharp trocar is then advanced through introducer 12, to pierce annulus fibrosus 122 and enter the disc. The trocar is then removed, and catheter 14 is advanced through a distal end of introducer 12, following the path created by the trocar in annulus fibrosus 122 beyond the end of the introducer. In such cases, the introducer is outside the annulus fibrosus and only the catheter with its guidable distal portion 16 is present inside the disc. The physician can manipulate the proximal portion 15 of the catheter to move the distal portion of the catheter to a selected location for applying thermal energy to the nucleus pulposus 120 or the inner wall 22 of annulus fibrosus 122.

Catheter 14 is not always pre-bent as shown in FIG. 3(a), but optionally can include a biased distal portion 28 if desired. "Pre-bent" or "biased" means that a portion of the catheter (or other structural element under discussion) is made of a spring-like material capable of assuming a bent structure in the absence of external stress but which, under selected stress conditions (for example, while the catheter is inside the introducer), can be held in a linear alignment. Such a biased distal portion can be manufactured from either spring metal or superelastic memory material (such as Tinel® nickel-titanium alloy, Raychem, Menlo Park Calif.), with the introducer (at least in the case of a spring-like material for forming the catheter) being sufficiently strong to resist the bending action of the biased tip and maintain the biased distal portion in alignment as it passes through the introducer. Compared to unbiased catheters, a catheter with a biased distal portion 28 encourages advancement of intradiscal section 16 substantially in the direction of the bend relative to other lateral directions as shown by the bent location of intradiscal section 16 indicated by dashed lines in FIG. 3(a). Biasing the catheter tip also further decreases likelihood that the tip 29 will be forced through the wall of the annulus fibrosus under the pressure being used to advance the catheter.

In addition to biasing a catheter tip prior to insertion into an introducer, a catheter tip can also be provided with mechanical deflecting means, such as a wire attached to one side of the tip that deflects the tip in the desired direction upon application of force to the proximal end of the deflection wire. Any device in which bending of the tip of a catheter of the invention is controlled by the physician is "actively steerable." In addition to a tip that is actively steerable by action of a wire, other forces for bending the tip can be used, such as hydraulic pressure and electromagnetic force (e.g., heating a shaped memory metal to cause it to contract). Any of a number of techniques can be used to provide selective bending of the catheter in one lateral direction.

Referring now to FIG. 5(a), a guiding mandrel 32 can be included both to add rigidity to the catheter and to inhibit movement of catheter 14 in the inferior and superior directions while positioned and aligned in the disc plane of a nucleus pulposus 120. This aids the functions of preventing undesired contact with a vertebra and facilitating navigation. The mandrel can be flattened to encourage bending in a plane (the "plane of the bend") orthogonal to the "flat" side of the mandrel. "Flat" here is a relative term, as the mandrel can have a D-shaped cross-section, or even an oval or other cross-sectional shape without a planar face on any part of the structure. Regardless of the exact configuration, bending will preferentially occur in the plane formed by the principal longitudinal axis of the mandrel and a line connecting the opposite sides of the shortest cross-sectional dimension of the mandrel (the "thin" dimension). To provide sufficient resistance to bending of the catheter out of the desired plane while encouraging bending in the desired plane, a minimum 1.25:1 ratio of "thickest" to "thinnest" cross-sectional dimensions along at least a portion of the intradiscal section. The maximum ratio is 20:1, with the preferred ratio being between 1.5:1 and 16:3, more preferably between 2:1 and 3.5:1. These ratios are for a solid mandrel and apply to any material being used, as deflection under stress for uniform solids is inversely proportional to the thickness of the solid in the direction (dimension) in which bending is taking place. For other types of mandrels (e.g., hollow or non-uniform), selection of dimensions and/or materials that provide the same relative bending motions under stress are preferred.

A catheter of the present invention is designed with sufficient torsional strength (resistance to twisting) to prevent undesired directional movement of the catheter. Mandrels formed from materials having tensile strengths in the range set forth in the examples of this specification provide a portion of the desired torsional strength. Other materials can be substituted according to operational functions as described in the examples and desired operating parameters.

While the mandrel can provide a significant portion of the column strength, selective flexibility, and torsional strength of a catheter, other structural elements of the catheter also contribute to these characteristics. Accordingly, it must be kept in mind that it is the characteristics of the overall catheter that determine suitability of a particular catheter for use in the methods of the invention. For example, a mandrel that does not have sufficient torsional strength when acting alone can be combined with another element, such as anti-twisting outer sheath 40 or inserting/advancing a second mandrel, to provide a catheter of the invention. Similarly, components found within the catheter, such as a heating element or potting compound, can be used to strengthen the catheter or provide directional flexibility at the locations of these elements along the catheter.

It is not necessary that the guiding mandrel 32 be flattened along its entire length. Different mandrels can be designed for different sized discs, both because of variations in disc sizes from individual to individual and because of variations in size from disc to disc in one patient. The bendable portion of the mandrel is preferably sufficient to allow intradiscal portion 16 of the catheter to navigate at least partially around the circumference of the inner wall of the annulus fibrosus (so that the operational functions of the catheter can be carried out at any desired location along the inner wall of the annulus fibrosus). Shorter bendable sections are acceptable for specialized instruments. In most cases, a flattened distal portion of the mandrel of at least 10 mm, preferably 25 mm, is satisfactory. The flattened portion can extend as much as the entire length of the mandrel, with some embodiments being flattened for less than 15 cm, in other cases for less than 10 cm, of the distal end of the guide mandrel.

In preferred embodiments the guide mandrel or other differential bending control element is maintained in a readily determinable orientation by a control element located at the proximal end of the catheter. The orientation of the direction of bending and its amount can be readily observed and controlled by the physician. One possible control element is simply a portion of the mandrel that extends out of the proximal end of the introducer and can be grasped by the physician, with a shape being provided that enables the physician to determine the orientation of the distal portion by orientation of the portion in the hand. For example, a flattened shape can be provided that mimics the shape at the distal end (optionally made larger to allow better control in the gloved hand of the physician, as in the handle of FIG. 3(b)). More complex proximal control elements capable of grasping the proximal end of the mandrel or other bending control element can be used if desired, including but not limited to electronic, mechanical, and hydraulic controls for actuation by the physician.

The guide mandrel can also provide the function of differential flexibility by varying the thickness of one or more dimensions (for example, the "thin" dimension, the "thick" dimension, or both) along the length of the mandrel. A guide mandrel that tapers (becomes gradually thinner) toward the distal tip of the mandrel will be more flexible and easier to bend at the tip than it is at other locations along the mandrel. A guide mandrel that has a thicker or more rounded tip than more proximal portions of the mandrel will resist bending at the tip but facilitate bending at more proximal locations. Thickening (or thinning) can also occur in other locations along the mandrel. Control of the direction of bending can be accomplished by making the mandrel more round, i.e., closer to having 1:1 diameter ratios; flatter in different sections of the mandrel; or by varying the absolute dimensions (increasing or decreasing the diameter). Such control over flexibility allows instruments to be designed that minimize bending in some desired locations (such as the location of connector of an electrical element to avoid disruption of the connection) while facilitating bending in other locations (e.g., between sensitive functional elements). In this manner, a catheter that is uniformly flexible along its entire length, is variably flexible along its entire length, or has alternating more flexible and less flexible segments is readily obtained simply by manufacturing the guide mandrel with appropriate thickness at different distances and in different orientations along the length of the mandrel. Such a catheter will have two or more different radii of curvature in different segments of the catheter under the same bending force.

In some preferred embodiments, the most distal 3 to 40 mm of a guide mandrel is thinner relative to central portions of the intradiscal section to provide greater flexibility, with the proximal 10 to 40 mm of the intradiscal section being thicker and less flexible to add column strength and facilitate navigation.

The actual dimensions of the guide mandrel will vary with the stiffness and tensile strength of the material used to form the mandrel. In most cases the mandrel will be formed from a metal (elemental or an alloy) or plastic that will be selected so that the resulting catheter will have characteristics of stiffness and bending that fall within the stated limits. Additional examples of ways to vary the stiffness and tensile strength include transverse breaks in a material, advance of the material so that it "doubles up," additional layers of the same or different material, tensioning or relaxing tension on the catheter, and applying electricity to a memory metal.

As illustrated in FIG. 5(b), in some embodiments of an apparatus of the invention, guiding mandrel is combined with at least a portion of the catheter 14 to form a structure which provides the functions of both, a wall/mandrel 41. In this figure, the wall/mandrel 41 of catheter 14 can be varied in dimensions as described in the previous section of this specification directed to a separate mandrel, with the same resulting changes in function. For example, changing the thickness of the wall/mandrel 41 changes the flexibility and preferred direction of bending of the catheter. In many cases, the wall/mandrel 41 will be thinner than other portions of the catheter wall 33 so that wall/mandrel 41 controls bending. Alternatively, wall/mandrel 41 can be formed of a different material than the other portions 33 of the catheter walls (i.e., one with a lower tensile strength and/or flexural resistance) to facilitate bending.

Returning now to FIG. 5(a), the guiding mandrel 32 is generally located in the interior of catheter 14, where it shares space with other functional elements of the catheter. For example and as shown in FIG. 5(a), thermal energy delivery device lumen 34 can receive any of a variety of different couplings from an energy source 20 to a thermal energy delivery device (functional element) further along the catheter, including but not limited to a wire or other connector between heating elements. Alternatively or concurrently, hollow lumens for delivery or removal of a fluid or solid connectors for application of a force to a mechanical element can be present, so no limitation should be placed on the types of force or material transporting elements present in the catheter. These are merely some of the possible functional elements that can be included in the intradiscal portion of the catheter. Accordingly, a general description will now be given of some of the possible functional elements.

Since the purpose of the inventive catheter is to apply thermal energy from a heating element to shrink disc tissue at a location inside a disc, a functional element is provided in or on the catheter for that purpose. Thermal energy delivery device is coupled to a thermal energy delivery source 20 through the catheter (see FIG. 3(a)). The functional element can be at any location in the intradiscal portion of the catheter, depending on its intended use. Multiple functional elements can be present, either multiple functional elements of different types (e.g., a heat source and a temperature sensor) or multiple functional elements of the same type (e.g., multiple resistive heat elements spaced along the intradiscal portion).

A variety of different types of thermal energy delivery devices can be used. In one embodiment, resistive heating element 18 is positioned proximal to the distal portion of intradiscal section 16 so that there is no substantial delivery of energy at the distal portion, which can then perform other functions without being constrained by being required to provide energy (or resist the resulting heat).

Some embodiments (e.g., FIG. 5(b)) have an interior infusion lumen 36. Infusion lumen 36 is configured to transport into the disc a variety of different mediums including but not limited to electrolytic solutions (such as normal saline or Ringer's lactate), contrast media, pharmaceutical agents, disinfectants, filling or binding materials such as collagen or cements, chemonucleolytic agents and the like. Further, infusion lumen 36 can be used as an aspiration lumen to remove nucleus material or excess liquid or gas (naturally present, present as the result of a liquefying operation, or present because of prior introduction) from the interior of a disc. When used to transport a fluid for irrigation of the disc, the infusion lumen is sometimes referred to as an irrigation lumen. Infusion lumen 36 can be coupled to medium reservoir 21 through the catheter (see FIG. 3(a)).

Included in the embodiment shown in this figure is one or more sensor lumens 42. An example is a wire connecting a thermal sensor at a distal portion of the catheter to a connector in the proximal handle 11 of the catheter.

Also included in the embodiment shown in FIG. 5(a) is an optional energy directing device 43 including but not limited to a thermal reflector or thermal insulator. Energy directing device 43 is configured to limit thermal energy delivery to a selected site of the disc and to leave other sections of the disc substantially unaffected by the delivered energy. Energy directing device 43 can be positioned on an exterior surface of catheter intradiscal section 16 and/or catheter 14 as well as in an internal portion of catheter intradiscal section 16 and/or catheter 14.

In one embodiment, catheter intradiscal section 16 and/or distal portion 28 are positionable to any selected site around and/or adjacent to inner wall 22 of annulus fibrosus 122 for the delivery of thermal energy. Intradiscal section 16 is navigational and can reach the posterior lateral 136, posterior medial 138, anterior medical and anterior lateral regions of the annulus fibrosus, as well as any selected area on or adjacent to inner wall 22. In a preferred embodiment, intradiscal section 16 is positioned adjacent to the entire posterior wall of the disc, particularly the posterior medial surface. Sufficient thermal energy can then be delivered, for example, to selectively heat the annulus to denervate pain receptors in the posterior annulus and adjacent to fissure 44, without undue damage to other portions of the intervertebral disc, particularly the nucleus.

Additionally, in one embodiment, intradiscal section 16 and/or distal portion 28 delivers thermal energy to reduce pain at a selected site (e.g., any portion of annulus fibrosus 122), without ablation or removal of any disc material adjacent to intradiscal section 16 of catheter 14 and with or without removal of water vapor from the disc but without charring the nucleus. Intradiscal section 16 also can heat the collagen component of the annulus, thereby shrinking the annulus, with or without desiccating local tissue.

In the preferred embodiment of FIG. 5(a), markings 38 are visible on the portion of the catheter that is located during normal operation outside the body being acted upon, particularly for embodiments in which the proximal end of the catheter is designed to be directly manipulated by the hand of the physician. Advancement of the catheter into the introducer will advance the markings into the introducer, thereby showing how far the catheter has been advanced into the nucleus. Such a visible marking 38 can be positioned on an exterior surface of the catheter or can be present on an interior surface and visible through a transparent outer covering or sheath. Preferred are markings every centimeter to aid the physician in estimating the catheter tip advancement.

If desired, visible markings can also be used to show twisting motions of the catheter to indicate the orientation of the bending plane of the distal portion of the catheter. It is preferred, however, to indicate the distal bending plane by the shape and feel of the proximal end of the catheter assembly. The catheter can be attached to or shaped into a handle 11 that fits the hand of the physician and also indicates the orientation of the distal bending plane. Both the markings and the handle shape thus act as control elements to provide control over the orientation of the bending plane; other control elements, such as plungers or buttons that act on mechanical, hydrostatic, electrical, or other types of controls, can be present in more complex embodiments of the invention.

Additionally, a radio-opaque marking device can be included in the distal portion of the catheter (such as in the tip or at spaced locations throughout the intradiscal portion) so that advancement and positioning of the intradiscal section can be directly observed by radiographic imaging. Such radio-opaque markings are preferred when the intradiscal catheter section is not clearly visible by radiographic imaging, such as when the majority of the catheter is made of plastic instead of metal. A radio-opaque marking can be any of the known (or newly discovered) materials or devices with significant radio-opacity. Examples include but are not limited to a steel mandrel sufficiently radio-opaque to be visible on fluoroscopy, a tantalum/polyurethane tip, a gold-plated tip, bands of platinum, stainless steel, or gold, soldered spots of gold, and polymeric materials with radio-opaque filler (such as barium sulfate). The resistive heater may provide sufficient radio-opacity in some embodiments to serve as a marking device.

A sheath 40 can optionally be positioned around catheter 14 (FIGS. 5(a) and (b)). Sheath 40 provides a flexible surface that is smooth and provides for easy introduction into a selected area within the disc. Sheath 40 can be made of a variety of different materials including but not limited to polyester, rayon, polyimide, polyurethane, polyethylene, polyamide and silicone. When visible markings are present to indicate the advancement of the catheter tip, either the sheath carries the markings, or the sheath is clear to reveal markings underneath.

Thermal energy delivery device 18 is typically a known resistive heating element. As illustrated in FIG. 6, a heating element 46 is positioned around an exterior of catheter 14. Heating element 46 is powered by a direct current source 20 (or, less preferably, alternating). Heating element 18 is made of a material that acts as a resistor. Suitable materials include but are not limited to stainless steel, nickel/chrome alloys, platinum, ferrite and the like.

Preferably, the heating element is inside the intradiscal section of catheter 14 (FIG. 8). The resistive material is electrically insulated and substantially no current escapes into the body. With increasing levels of current (most easily controlled by controlling the applied voltage), element 46 heats to greater temperature levels. Additionally, a circuit can be completed substantially entirely at intradiscal section 16 and a controllable delivery of thermal energy is achieved. In one embodiment, 2 watts pass through heating element 46 to produce a temperature of about 55° C. in a selected target such as fissure 44, 3 watts produces 65° C., 4 watts produces 75° C., and so on.

In preferred embodiments of the invention, sufficient energy is delivered to intervertebral disc to heat but not ablate tissue positioned adjacent to catheter 14. The amount of thermal energy delivered is a function of (i) the amount of current passing through heating element 46, (ii) the length, shape, and/or size of heating element 46, (iii) the resistive properties of heating element 46, and (iv) the gauge of heating element 46. Power supply 20 associated with heating element 46 may be battery based. Energy source 20 and catheter 14 may both be entirely or only partially disposable.

Referring now to FIG. 7, a thermal sensor 48 may be positioned in an interior location of catheter 14. In another embodiment, thermal sensor 48 is positioned on an exterior surface of catheter 14. A thermal sensor can be used to control the delivery of energy to thermal energy delivery device 18. A potting material can be used to fix the position of thermal sensor 48 and provide a larger area from which to average the measured temperature. Thermal sensor 48 is of conventional design, including but not limited to a thermistor; T type thermocouple with a copper constantan junction; J type, E type, or K type thermocouples; fiber optics; resistive wires; IR detectors; and the like. Optionally, there may be a lumen 42 for the thermal sensor connections.

As illustrated in FIG. 8, sheath 40 may be used to cover resistive heating element 46. A plurality of resistive heating elements 46 can be used (FIG. 9) in a catheter of the invention.

Referring now to the embodiment shown in FIG. 10, thermal energy delivery device 18 comprises one or more resistive heating elements 46 coupled to a current source. Resistive heating elements 46 are positioned along intradiscal section 16 at locations where they controllably deliver thermal energy to selected structures, including but not limited to pain receptors in a fissure 44. Resistive heating elements 46 can be segmented and multiplexed so that only certain resistive heating elements, or combinations of resistive heating elements are activated at any one particular time. Thermal sensor 48 can be positioned between resistive heating elements 46 and/or at an exterior or interior location of catheter 14. In the embodiment illustrated in FIG. 10, catheter 14 can be prepared with a wound helical structural element 49 to increase flexibility and minimize kinking. However, other structures and geometries are suitable for catheter 14, including but not limited to a substantially smooth surface (and specifically including the devices using an internal guide mandrel as previously described). For example, a sheath can be provided over the heating element, and the guiding mandrel inside the coil can be encapsulated in silicone potting material. The tubing flexibility and the silicone potting material prevent kinking. Additionally, sheath 40 can be positioned around catheter 14 and also around resistive heating elements 46 to afford a substantially smooth surface. Resistive heating elements 46 can be at least partially covered by a thermally insulating material, for example, along one side of the catheter, to selectively heat disc tissue on the opposite side.

Referring now to FIGS. 11 and 12, an open or closed loop feedback system 52 couples sensors 48 to energy source 20.

The temperature of the tissue or of element 46 (FIG. 10) is monitored by sensors 48, and the output power of energy source 20 adjusted accordingly. The physician can, if desired, override control system 52. A microprocessor can be included and incorporated in the closed or open loop system to switch power on and off, as well as to modulate the power. The closed loop system utilizes a microprocessor to serve as a controller 54 which acts to monitor the temperature and adjust the power accordingly. Alternatives to the microprocessor are, for example, analog control circuitry and a logic controller.

With the use of sensors 48 and feedback control system 52, a tissue adjacent to resistive heating elements 46 can be maintained at a desired temperature for a selected period of time without aberrant high temperature fluctuations. Each resistive heating element 46 can be connected to control and power supply resources which generate an independent output for each resistive heating element 46. For example, a desired thermal output can be achieved by maintaining a selected energy at resistive heating elements 46 for a selected length of time.

When a resistive heating element 46 is used, current delivered through resistive heating element 46 can be measured by current sensor 56. Voltage can be measured by voltage sensor 58. Resistance and power are then calculated at power calculation device 60. These values can then be displayed at user interface and display 62. Signals representative of power and resistance values are received by a controller 54.

A control signal is generated by controller 54 that is related to current and voltage. The control signal is used by power circuits 66 to adjust the power output in an appropriate amount in order to maintain the desired power delivered at respective resistive heating elements 46.

In a similar manner, temperatures detected at sensors 48 provide feedback for maintaining a selected power. The actual temperatures are measured at temperature measurement device 68, and the temperatures are displayed at user interface and display 62. A control signal is generated by controller 54 that is related to the actually measured temperature and a desired temperature. The control signal is used by power circuits 66 to adjust the power output in an appropriate amount in order to maintain the desired temperature delivered at the respective sensor 48. A multiplexer can be included to measure current, voltage, and temperature at the sensors 48, so that appropriate energy can be delivered to resistive heating elements 46.

Controller 54 can be a digital or analog controller or a computer with software. When controller 54 is a computer, it can include a CPU coupled through a system bus. Included in this system can be a keyboard, a disc drive or other non-volatile memory system, a display, and other peripherals, as are known in the art. Also coupled to the bus can be a program memory and a data memory.

User interface and display 62 includes operator controls and a display. Controller 54 can be coupled to imaging systems well known in the art.

The output of current sensor 56 and voltage sensor 58 is used by controller 54 to maintain a selected power level at resistive heating elements 46. A predetermined profile of power delivered can be incorporated in controller 54, and a preset amount of energy to be delivered can also be profiled.

Circuitry, software, and feedback to controller 54 result in process control and in the maintenance of the selected power that is independent of changes in voltage or current. Control can include (i) the selected power and (ii) the duty cycle (wattage and on-off times). These process variables are controlled and varied while maintaining the desired delivery of power independent of changes in voltage or current, based on temperatures monitored at sensors 48.

In the embodiment shown in FIGS. 11 and 12, current sensor 56 and voltage sensor 58 are connected to the input of an analog amplifier 70. Analog amplifier 70 can be a conventional differential amplifier circuit for use with sensors 48, 56 and 58. The output of analog amplifier 70 is sequentially connected by an analog multiplexer 72 to the input of A/D converter 74. The output of analog amplifier 70 is a voltage which represents the respective sensed parameters. Digitized amplifier output voltages are supplied by A/D converter 74 to microprocessor 54. Microprocessor 54 may be a type 68HCII available from Motorola. However, it will be appreciated that any suitable microprocessor or general purpose digital or analog computer can be used to control the parameters of temperature, voltage or current.

Microprocessor 54 sequentially receives and stores digital representations of sensed parameters. Each digital value received by microprocessor 54 corresponds to different parameters. Calculated power and temperature values can be indicated on user interface and display 62. Alternatively, or in addition to the numerical indication of power, calculated power values can be compared by microprocessor 54 with power limits. When the values exceed predetermined power or temperature values, a warning can be given on user interface and display 62, and additionally, the delivery of electromagnetic energy can be reduced, modified or interrupted. A control signal from microprocessor 54 can modify the power level supplied by energy source 20.

In preferred embodiment of the invention, the materials that make up the various parts of an apparatus of the invention have the following characteristics:

    __________________________________________________________________________             Tensile                        Geometry (height,                               Strength                                                                            %    Conductivity                                                                            Resistivity                                                                          Melt temp.                                                                           width, and/or dia.)                     Component                                                                              in MPa                                                                              Elongation                                                                          cal/cm.sup.2 /cm/sec/° C.                                                        nΩ*m                                                                           ° C.                                                                          in mm                                   __________________________________________________________________________     Mandrel 600-2100                                                                            5-100                                                                               N/A      N/A   N/A   height 0.2-2.3                                                                 width 0.05-0.5                          Heating Element                                                                        300 min.                                                                            20 (min.)                                                                           .025-0.2 500-1500*                                                                            N/A   0.05-0.5 dia.                           Conductor Wire                                                                         N/A  N/A   .2-1.0  150 max.*                                                                            N/A   0.1-0.5 dia.                            Plastic sheath                                                                         N/A  25 (min.)                                                                           N/A      N/A   80° (min.)                                                                    0.05-0.02 thickness                     __________________________________________________________________________

Another preferred characteristic is that the minimum ratio of heating element resistivity to conductor wire resistivity is 6:1; the preferred minimum ratio of guiding mandrel height to guiding mandrel width is 2:1. Tensile strength and % elongation can be measured according to ASTME8 (tension test of metallic materials). Conductivity and resistivity can be determined by procedures to be found in ASTM Vol 2.03 for electrothermal properties.

A particularly preferred embodiment of a catheter of the invention can be prepared using a covering sheath of polyimide with an outside diameter of 1 mm and a wall thickness of 0.05 mm. Such a sheath provides a significant fraction of the stiffness and torsional strength appropriate for the catheter. Internal to the polyimide sheath and in the intradiscal section of the catheter is a heating coil of insulated nickel/chromium wire that has an outside diameter that matches the interior diameter of the polyimide sheath. This heating coil provides both heat and additional stiffness to the assembly. Also internal to the polyimide sheath on each side of the coil (longitudinally) is a 0.10 mm-walled, 304 stainless-steel, metallic band whose outer diameter matches the inner diameter of the sheath, the distal band having a hemispherical end that exits the end of the polyimide sheath and creates a blunt tip 29 at the end of the catheter. These bands provide enhanced radio-opacity for fluoroscopic visualization, as well as some of the stiffness of the assembled apparatus. Proximal to the proximal metallic band and internal to the sheath is an additional polyimide tube 47 that increases the stiffness of the catheter in the region that transitions from the intradiscal section containing the coil to the rigid proximal section. Proximal to the second polyimide tube and internal to the sheath is a 304 stainless steel (fully hard) hypodermic tube with an outside diameter matching the inside diameter of the polyimide sheath and a wall thickness of 0.1 mm. This combination provides the rigidity needed for a physician to advance the distal portion of the device inside a nucleus pulposus and provides tactile force feedback from the tip to the physician.

In some embodiments, inside the bands, coil, hypodermic tube, and both the polyimide sheath and internal polyimide tube is a guiding mandrel 32 that extends from a proximal handle to the tip of the catheter. In one embodiment, this mandrel is 0.15 mm by 0.05 mm and formed from 304 stainless steel. In another embodiment, it is a 0.3 mm diameter 304 stainless steel wire, with the distal 2.5 cm flattened to 0.02 mm by 0.5 mm.

Inside the center of the heating coil is a T-type thermocouple potted with cyanoacrylate adhesive into a polyimide sheath and located alongside the mandrel. The thermocouple wire travels through the coil and hypodermic tube to the handle at the proximal end of the apparatus. Two copper conductor wires (36 gauge insulated with polyimide) are soldered to the heating coil and pass through the hypodermic tube and handle to the proximal handle electrical connector, which allows a power supply and feedback controls to be connected to electrical elements in the catheter. One embodiment has the handle fitted with a 1-3-meter cable extension ending in an electrical connector to eliminate the need for a connector in the handle. This design reduces weight (from connector elements) on the proximal end and increases the amount of tactile feedback that a physician obtains while navigating the device.

The entire inside of the catheter in one embodiment is encapsulated with a silicone material which removes air (which would insulate the heat created by the heating coil) and helps support the polyimide sheath to prevent collapse (i.e., increases stiffness). Instead of the silicone, another embodiment uses an epoxy which remains flexible when cured. Strain relief is provided between the catheter body and the handle with an elastomeric boot. The distal end of the catheter is pre-bent 15-30° off the longitudinal axis of the catheter at about 5-10 mm from the distal tip.

The catheter in one embodiment carries visible markings 38 internally on the hypodermic tube (with the markings being visible through the polyimide sheath) to indicate distance of insertion of the catheter into an introducer and/or distance that the distal end of the atheter extends out of the introducer into a disc. The catheter is also marked both visually and with tactile relief on its handle to indicate the direction of bending of the pre-bent tip and biased stiffness.

The guidable apparatus described herein can be used in any of a number of methods to treat local areas within a disc. Several specific methods that can be carried out with an apparatus of the invention will now be described.

Degenerative discs with tears or fissures can be treated with or without the removal of disc tissue other than limited desiccation of the nucleus pulposus which reduces its water content, but not in a manner which would shorten the disc space and cause instability. Tears and fissures also can be ameliorated by shrinking the collagen component of the annulus to bring the sides of the tear closer together into their normal position, creating a smaller annular circumference. To do that, the disc adjacent to the fissure is typically heated to a temperature in the range of about 45-70° C. Thermal shrinking of collagen also facilitates ingrowth of more collagen, which further increases annular stiffness. Fissures can also be repaired with sealants, that is, bonding material (adhesives and cements) and filler (non-adhesive material that blocks the opening) which help seal the fissure.

In one aspect of the invention, sealants such as collagen, albumin, and a combination of fibrinogen and thrombin are delivered to the fissure. Collagen from a variety of sources can be used (e.g., bovine extracted collagen from Semex Medical, Frazer Pa., or human recombinant from Collagen Corp., Palo Alto Calif.). The collagen is injected dissolved or as fine slurry, after which it is allowed to set or may be heated in situ in the fissure, where it provides a matrix for normal collagen deposition by the body.

A variety of different biomaterials can also be delivered to the fissure, including but not limited to irrigating solutions (normal saline, 5% glucose, etc.), contrast media (e.g. Conray meglumine iothalamate), pharmaceutical agents (e.g. steroids like methylprednisolone sodium succinate, Pharmacia Upjohn, Kalamazoo Mich., and non-steroidal anti-inflammatory agents), chemonucleolytic enzymes (e.g., chymopapain), hydrogel (such as disclosed in U.S. Pat. No. 4,478,822), osteoinductive materials (e.g., BMP (see U.S. Pat. No. 5,364,839), chondrocyte inductive material (e.g.,TGF-β), and the like. The materials are delivered via the catheter and/or introducer to the disc. Preferably, however, when precision placement of the material (as in a fissure) is necessary or desired, the preferred delivery method would use the apparatus described above, especially when delivery to the posterior, posterior lateral, or posterior central region of the disc is desired. The materials may be administered sequentially, such as beginning with an irrigating solution which helps the physician view the pathology, and following with products to seal a fissure.

The materials are delivered in an amount sufficient to decrease the extent of the fissure at least partially, preferably to fill the fissure completely. The delivered material can be fixed in position with an adhesive, a hydrogel that is liquid at room temperature but gels at body temperature, as a result of naturally occurring processes (such as interaction of fibrinogen and thrombin) within the disc, or by heating the disc as described in more detail below.

To seal a fissure, a combination of thrombin and fibrinogen is injected at the fissure, after which it coagulates and seals the tissue. A kit with appropriate syringes and other equipment is available from Micromedics, Inc., Eagan, Minn. Frozen fibrinogen solution is thawed in its plastic bag and then dispensed to a small med cup. Thrombin is reconstituted with sterile water in the "slow gel" concentration (100 units/ml) for tissue bonding. For example, 100 ml is added to a vial containing 10,000 units. Thrombin solution is withdrawn from the vial and dispensed to a second med cup. The two syringes are filled equally, one with each solution. Then the syringe tips are each twisted into an applicator that mixes the solutions before passing them to an administration tube. The syringes are fitted into the dual syringe holder and the plunger link, which helps the practitioner administer equal amounts of thrombin and fibrinogen. Then the practitioner connects the administration tube to the proximal end of the inventive catheter, depresses the plungers and dispenses the sealant solution to the fissure. The thrombin and fibrinogen react and form a natural seal over the fissure.

Chymopapain can be injected through the subject catheter, particularly near a herniation of the disc. Chymopapain splits side chains off proteoglycan molecules, thereby decreasing their ability to hold onto water. The disc gradually decreases in size. A typical dose is 0.75 to 1.0 ml (2000 pKat/ml).

Resistive heating is delivered to a selected section of the disc in an amount which does not create a destructive lesion to the disc, other than at most a change in the water content of the nucleus pulposus. In one embodiment there is no removal and/or vaporization of disc material positioned adjacent to the resistive heater in a nucleus pulposus. Sufficient thermal energy is delivered to change the disc's biochemical and/or biomechanical properties without structural degradation of tissue. Neurophysiologic modifications include denervation of nociceptors in a tear or fissure in the annulus fibrosus and inactivation of neurotransmitters. Heat-sensitive enzymes also are inactivated.

Degenerative intervertebral discs with fissures are treated by denervating nerves imbedded in the interior wall of the annulus fibrosus as well as nerves in the annulus and its exterior surface. Resistive heating is used to selectively cauterize granulation tissue which is pain sensitive and forms in a tear or fissure.

Controlled thermal energy also breaks down heat-sensitive enzyme systems and neurotransmitters that generate pain both within the disc and external to the disc when they migrate out through a fissure. Generally, these enzymes and neurotransmitters only work within small ranges of pH and temperature at or near normal pH and temperature. Therefore, the application of heat inactivates some of these proteins.

Controlled thermal energy is applied to heat and shrink the collagen component of the annulus fibrosus. This reduces the redundancy in the disc roll that is created in a degenerative disc. Delivery of heat to the nucleus pulposus may optionally be used to remove some water (desiccate the nucleus) and permit the nucleus pulposus to withdraw. This reduces a "pushing out" effect that created a contained herniation. The combination of shrinking of the nucleus pulposus by desiccation and tightening up the annulus fibrosus wall rejuvenates the disc. Reducing the pressure in the disc and tightening the annulus fibrosus help stabilize the spine and relieve pain. Application of controlled heat locally increases the stiffness of the disc in that locale, which degree of stiffness can be selected to advantage using the fine temperature and positional control afforded by the present invention.

All publications and patent applications mentioned in this specification are herein incorporated by reference to the same extent as if each individual publication or patent application was specifically and individually indicated to be incorporated by reference. The foregoing description of preferred embodiments of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many modifications and variations will be apparent to practitioners skilled in this art. It is intended that the scope of the invention be defined by the following claims and their equivalents. 

What is claimed is:
 1. An externally guidable intervertebral disc apparatus for manipulation of tissue at a selected location of an intervertebral disc, the apparatus comprising:a proximal end for externally guiding a distal end of the apparatus within an intervertebral disc; a self-navigating intradiscal section adjacent a distal end of the apparatus which navigates itself adjacent an inner wall of an annulus of the disc when the apparatus is extended into an intervertebral disc, the intradiscal section having (a) sufficient rigidity to be advanceable through a nucleus pulposus and around the inner wall of an annulus fibrosus under a force applied longitudinally to the proximal end of the apparatus, (b) insufficient penetration ability to be advanceable out through the annulus fibrosus under the applied force, and (c) sufficient flexibility in a direction of a disc plane to be compliant with the inner wall; and a resistive energy delivery device incorporated into the apparatus adjacent the intradiscal section for preferentially heating the selected location of the disc.
 2. The apparatus of claim 1, wherein at least a portion of the intradiscal section further has differential bending ability in two orthogonal directions at right angles to the longitudinal axis.
 3. The apparatus of claim 2, wherein the differential bending ability is controlled by a structural element with different thicknesses in the two orthogonal directions.
 4. The apparatus of claim 2, wherein the differential bending ability is maintained in a fixed orientation relative to the apparatus by a control element at the proximal end of the apparatus.
 5. The apparatus of claim 1, wherein the intradiscal section has a length at least one-half the diameter of the nucleus pulposus.
 6. The apparatus of claim 1, wherein the intradiscal section has a distal portion with a blunt tip.
 7. The apparatus of claim 1, wherein the intradiscal distal tip has lower rigidity than a majority of the intradiscal section.
 8. The apparatus of claim 1, wherein the intradiscal section has a directionally biased distal tip portion.
 9. The apparatus of claim 1, wherein an electrical power supply is operably attached to the resistive energy delivery device through the apparatus.
 10. The apparatus of claim 1, wherein the apparatus includes a plurality of resistive energy delivery devices.
 11. The apparatus of claim 1, wherein intradiscal section has at least one sensor capable of monitoring temperature, power, voltage, or a combination thereof, and thereby controlling heating.
 12. The apparatus of claim 11, wherein information from the sensor controls power delivered to the resistive energy delivery device.
 13. The apparatus of claim 1, wherein at least a portion of the apparatus is radiographically opaque.
 14. The apparatus of claim 1, wherein the intradiscal section of the apparatus is configured to have at least two radii of curvature when advanced around the annulus.
 15. The apparatus of claim 1, wherein the apparatus includes an irrigation lumen extending from a proximal end of the apparatus to the intradiscal section.
 16. The apparatus of claim 1, wherein intradiscal section of the apparatus has one or more flat sides.
 17. The apparatus of claim 1, wherein the resistive energy delivery device is located along one side of the intradiscal section.
 18. The apparatus of claim 1, wherein the apparatus further comprises an introducer capable of entering the disc and through which the intradiscal section of the apparatus is capable of sliding.
 19. The apparatus of claim 1, wherein the apparatus further comprises an active steering mechanism.
 20. The apparatus of claim 1, wherein the intradiscal section comprises a helical support structure.
 21. The apparatus of claim 1, wherein the intradiscal section has a bending stiffness as measured in Taber stiffness units between about 2-400 units in a desired bending plane.
 22. The apparatus of claim 1, wherein the intradiscal section has a bending stiffness as measured in Taber stiffness units between about 3-150 units in a desired bending plane.
 23. The apparatus of claim 1, wherein the intradiscal section has a bending stiffness as measured in Taber stiffness units between about 4-30 units in a desired bending plane.
 24. The apparatus of claim 23, wherein the proximal end of the apparatus has a column strength between about 0.2-7 kg.
 25. The apparatus of claim 23, wherein the proximal end of the apparatus has a column strength between about 0.7-4 kg.
 26. The apparatus of claim 1, wherein the intradiscal section has a column strength between about 0.05-4 kg.
 27. The apparatus of claim 26, wherein the proximal end of the apparatus has a column strength between about 0.1-25 kg.
 28. The apparatus of claim 26, wherein the proximal end of the apparatus has a column strength between about 0.2-7 kg.
 29. The apparatus of claim 26, wherein the proximal end of the apparatus has a column strength between about 0.7-4 kg.
 30. The apparatus of claim 1, wherein the intradiscal section has a column strength between about 0.05-2 kg.
 31. The apparatus of claim 30, wherein the proximal end of the apparatus has a column strength between about 0.1-25 kg.
 32. The apparatus of claim 30, wherein the proximal end of the apparatus has a column strength between about 0.2-7 kg.
 33. The apparatus of claim 30, wherein the proximal end of the apparatus has a column strength between about 0.7-4 kg.
 34. The apparatus of claim 1, wherein the resistive energy delivery device is capable of delivering a controlled amount of energy such that no vaporization occurs when energy is delivered by the resistive energy delivery device.
 35. The apparatus of claim 1, wherein the resistive energy delivery device is capable of delivering a controlled amount of energy at such that no material other than water is removed when energy is delivered by the resistive energy delivery device.
 36. The apparatus of claim 1, wherein the resistive energy delivery device is capable of delivering a controlled amount of energy such that no destructive lesion is formed on a disc when energy is delivered by the resistive energy delivery device.
 37. An intervertebral disc apparatus, comprising:a catheter including a self-navigating intradiscal section with a resistive energy delivery device, wherein the intradiscal section has sufficient column strength to be advanceable through a nucleus pulposus of an intervertebral disc and to be navigable along an inner wall of an annulus fibrosus, a low enough flexural strength in an axial plane of the intervertebral disc, and a sufficient flexural strength in oblique and cephalo-caudal planes to be constrained within the axial plane. 